Image sensors have become ubiquitous. They are widely used in digital still cameras, cellular phones, security cameras, medical, automobile, and other applications. The technology used to manufacture image sensors, and in particular CMOS image sensors, has continued to advance at great pace. For example, the demands of higher resolution and lower power consumption have encouraged the further miniaturization and integration of the image sensor.
As the pixels become smaller, the surface area that can receive incident light is also reduced. The pixel typically has a light-sensing element, such as a photodiode, which receives incident light and produces a signal in relation to the amount of incident light. Thus, as the pixel area (and thus the photodiode area) decreases, the pixel has a lower sensitivity and lower signal saturation level.
The above mentioned low saturation level is primarily a result of the low well capacity of a small photodiode. One prior art solution for this problem is to increase the impurity concentrations of the layers comprising the photodiode. For example, the commonly used pinned photodiode has a structure that is an N-type layer surrounded by a P or P+ region. However, increasing the impurity concentration of the N-type layer tends to cause an increase of image lag.
The present invention is directed towards a photodiode and pixel design that has high well capacity and with limited image lag.